1. Field of the Invention
The present invention relates to an inkjet printer.
2. Discussion of the Background
Conventionally, there is known an inkjet printer in which ink is ejected from a print head, which is disposed to face a platen, onto a print medium put on the platen while reciprocating the print head in a left-right direction so as to print the print medium. As one of such inkjet printers, there is a printer of a type ejecting ultraviolet curable ink (hereinafter, referred to as UV ink) having a property that it is cured when irradiated with ultraviolet light. Since the UV ink has excellent weather resistance and excellent water resistance, the UV ink allows printed matters to be used as outdoor advertising posters or the like. Therefore, the UV ink has the advantage that the printed matters can be used for various purposes as compared to printed matters printed with water-soluble ink.
By the way, an inkjet printer of a type ejecting UV ink to print is provided with an ultraviolet light irradiation device for irradiating the UV ink deposited on the print medium with ultraviolet light to cure the UV ink. In recent years, an inkjet printer has been developed in which an ultraviolet light emitting diode (hereinafter, referred to as UVLED) is used as a light source for emitting ultraviolet light in the ultraviolet light irradiation device (see, for example, JP-A-2004-188920).
As shown in FIG. 11(a), a conventional print unit 500 has a right ultraviolet light irradiation device 520R and a left ultraviolet light irradiation device 520L which are disposed in pairs on the right and left sides of the print head 510 and in which UVLEDs are arranged. The right ultraviolet light irradiation device 520R and the left ultraviolet light irradiation device 520L are adapted to emit ultraviolet light toward the print medium 501 located below the print unit 500. For ease of explanation, the following description will be made according to definition that directions shown by arrows shown in FIG. 11(a) are the forward, backward, leftward, and rightward directions, respectively. The print head 510 includes, for example, a magenta print head 510M for ejecting magenta UV ink droplets from a plurality of nozzles (not shown) formed in the bottom toward the print medium 501, an yellow print head 510Y for ejecting yellow UV ink droplets similarly, a cyan print head 510C for ejecting cyan UV ink droplets similarly, and a black print head 510K for ejecting black UV ink droplets similarly.
To conduct printing on a printing line 508 of the print medium 501, UV ink droplets are ejected from the respective nozzles of the print head 510 so that the UV ink droplets are superposed in predetermined patterns on a printing line 508 while reciprocating the print unit 500 above the printing line 508 a predetermined number of passes. During this, the right ultraviolet light irradiation device 520R and the left ultraviolet light irradiation device 520L emit ultraviolet light of strength capable of completely curing UV ink. The printing line 508 is irradiated with the ultraviolet light so as to cure the UV ink deposited on the printing line 508. In this manner, the printing is conducted.
FIGS. 11(b) and 11(c) are sectional views showing states that UV ink droplets ejected from nozzles are deposited on the printing line 508 on the way of printing on the printing line 508. FIG. 11(b) shows a state that uncured UV ink droplets 512 are ejected at the current pass and deposited on completely cured UV ink droplets 511 which were ejected at the last pass and deposited on the printing line 508 and which were irradiated with ultraviolet light and thus completely cured. Since the UV ink droplets 511 are completely cured, the affinity of the uncured UV ink droplets 512 for the completely cured UV ink 511 are poor so that the uncured UV ink droplets 512 are deposited in a raised shape like beading because of surface tension. After the uncured UV ink droplets 512 are deposited in a beading state, the uncured UV ink droplets 512 spread very little until irradiation with ultraviolet light because of poor affinity and is then completely cured in this state by irradiation with ultraviolet light.
On the other hand, FIG. 11(c) shows a state that uncured UV ink droplets 514 are ejected at the current pass and deposited on uncured UV ink droplets 513 which were ejected at the last pass and deposited on the printing line 508 and which were not cured (cured very little). The affinity of the later uncured UV ink droplets 513 for the prior uncured UV ink droplets 514 are good so that, after the later uncured UV ink droplets 513 are deposited in a beading state, the later uncured UV ink droplets 513 are mixed with the prior uncured UV ink droplets 514 and thus bleed. The later uncured UV ink droplets 514 and the prior uncured UV ink droplets 513 are mixed so as to form a mixed UV ink 515. The mixed UV ink 515 is irradiated with ultraviolet light and is thus completely cured.
By the way, for printing on the print medium 501 by the print unit 500, it is preferable that UV ink droplets deposited and superposed on the print medium 501 are not mixed and thus do not bleed, but the UV ink droplets spread and are thus leveled. In this case, the print medium 501 with desired printing (desired printed matter) can be obtained. However, when the uncured UV ink droplets 512 are superposed on and adhere to the completely cured UV ink droplets 511 as shown in FIG. 11(b), the completely cured UV ink droplets 511 and the uncured UV ink droplets 512 are not mixed and thus do not bleed. However, the completely cured UV ink droplets 511 reject the uncured UV ink droplets 512 so that the uncured UV ink droplets 512 may be cured by irradiation with ultraviolet light in the state remaining a raised shape like beading on the surface of the completely cured UV ink droplets 511. As compared to the desired printed matter, the printed matter in which UV ink droplets are cured in the state remaining the beading shape may have poorer print quality because reflection of light from the printed matter may differ so as to cause difference in vision (optical fringes).
When the later uncured UV ink droplets 514 are superposed on and adhere to the prior uncured UV ink droplets 513 as shown in FIG. 11(c), the later uncured UV ink droplets 513 may be mixed with the prior uncured UV ink droplets 514 and thus bleed so that the UV ink droplets may be cured by irradiation with ultraviolet light in the mixed and bleeding state. As compared to the desired printed matter, the printed matter in which UV ink droplets are cured in the mixed and bleeding state may have poorer print quality because a mixed and bleeding portion of the printed matter has different color in vision.
In addition, some UV inks have a feature that the volume of the ink itself is reduced (i.e. contracts) while being cured by irradiation with ultraviolet light. If using UV ink having such contraction feature, a phenomenon that a cured portion of ink attracts uncured portion of ink around the cured portion is caused because of the contraction. This phenomenon produces indented patterns (curing fringes) on the surface of the printed matter and is thus one of leading causes of optical fringes.
Although there is a problem of poor print quality because the UV ink droplets are cured in the state remaining a beading shape or the UV ink droplets are cured in the mixed and bleeding state, higher printing speed is sometimes required rather than the print quality depending on the applications of printed matters.